The following processes are performed when an antenna installed in a transmitting/receiving apparatus such as a hand-held terminal apparatus is designed.
First, an antenna model which does not include a matching circuit is created, and antenna characteristics such as the antenna impedance and the voltage standing wave ratio (VSWR) of the created antenna model are calculated in a simulation.
Next, it is determined whether or not the voltage standing wave ratio of the calculated antenna model satisfies a desired standard.
When it is determined that the desired standard is not satisfied, a model of a mating circuit is created with reference to the calculated antenna impedance, and the created matching circuit is added to the antenna model. For the antenna model to which the matching circuit has been added, an antenna characteristic such as a voltage standing wave ratio is calculated in a simulation, and it is evaluated whether or not the calculated antenna characteristic satisfies the desired standard.
When it is evaluated that the calculated antenna characteristic satisfies the desired standard, the designing of the antenna ends. By contrast, when it is evaluated that the calculated antenna characteristic does not satisfy the desired standard, the process returns to the configuring of a matching circuit, and the antenna designing continues.
A prior art exists wherein the radiation characteristic of a patch antenna is measured using predetermined parameters, including, for example, the conductivity of a transparent conductive film that functions as a radiating element and the conductivity of a microstripline.
A prior art exists wherein the radiant efficiencies of a matching element and an antenna of a transmission and/or reception module are measured.
A prior art exists wherein, for a notch antenna including a reactive circuit, a relationship between a frequency and a return loss and a relationship between a frequency and an antenna efficiency are determined using an electromagnetic simulation such as a Finite Difference Time Domain Method (FTDT).
A prior art exists wherein a relationship is examined between the dielectric constant of a dielectric space between a radiation antenna element and a feedline and the radiant efficiency of a microstrip slot antenna.
A prior art exists wherein the radiation resistance of a dipole antenna system is calculated from an approximation formula using radiation resistance, inductive reactance, capacitive reactance, ohmic feed point ground loss, and a skin effect.
A prior art exists wherein the radiant efficiency of a loop antenna is calculated from the radiation resistance of the loop antenna and the resistance loss of a conductor forming the loop antenna.
In recent years, the size and thickness of hand-held terminal apparatuses have been decreased, thereby decreasing the size and thickness of a space for installing an antenna to be provided for the hand-held terminal apparatuses. As a result, when, for example, a hand-held terminal apparatus including a sliding mechanism for slidably attaching a cover to the body of the hand-held terminal uses a metal as the sliding mechanism, the antenna might be located near the metal. If a metal is near an antenna, a current that cancels an antenna current will flow through the metal and the antenna performance will thus be degraded.
Performance measures of an antenna include radiation resistance Rr. Assuming that loss resistance included in, for example, an antenna, a matching circuit, and a feedline is Rl, radiation efficiency η, which is the ratio between actual electric power applied to the antenna and electric power radiated from the antenna, is expressed by the following formula.
                    η        =                              R            r                                              R              r                        +                          R              1                                                          [                  Formula          ⁢                                          ⁢          1                ]            
As is clear from formula (1), when radiation resistance Rr of the antenna is small, radiation efficiency η is degraded remarkably even if loss resistance Rl is a small value. Accordingly, the antenna is desirably designed in such a manner that radiation resistance Rr becomes large.
However, in the case of a thin and small hand-held terminal apparatus, since a metal could be close to the antenna as described above, it could be difficult to radiate radio waves, i.e., radiation resistance Rr could become small. Accordingly, in order to design an antenna with a small radiation resistance Rr, attention needs to be paid to loss resistance Rl even when this loss resistance Rl is a small value.
In the case of an antenna with a low radiation resistance Rr, when input impedance deviates from a characteristic impedance (e.g., 50Ω), matching needs to be achieved by a matching circuit.
A matching element forming the matching circuit includes a small number of resistance components in addition to a capacitance component or an inductance component. Accordingly, when the matching circuit is provided between an antenna and a transmitter and receiver module and when a current flows through the matching circuit, the resistance components of the matching element forming the matching circuit could form loss resistance Rl. As a result, when radiation resistance Rr of the antenna is small, even if matching is achieved by the matching circuit, an antenna characteristic such as radiation efficiency η of the antenna could possibly not satisfy a standard due to the influence of the resistance components of the matching element.
Accordingly, in order to design an antenna with a small radiation resistance Rr, antenna characteristics need to be calculated in consideration of a loss resistance component in addition to a capacitance component or inductance component and a parasitic inductance component or parasitic capacitance component of each matching element forming a matching circuit.
In this regard, when the aforementioned conventional antenna designing process is performed in consideration of a loss resistance component of a matching element, the following processes are performed.
First, a model of each component of a matching element forming a matching circuit is individually created in such a manner that the model includes a parasitic capacitance component or parasitic inductance component and a loss resistance component in addition to a capacitance component or inductance component. The value of each component of the created matching element is individually input and set. An antenna characteristic of an antenna model including the created matching circuit is calculated in a simulation.
Next, it is evaluated whether or not the calculated antenna characteristic satisfies a desired standard. When it is evaluated that the calculated antenna characteristic does not satisfy the desired standard, the process returns to the creating of the model of each component of the matching element forming the matching circuit, and the antenna designing process continues.
As described above, in the conventional antenna designing process, for a matching element forming a matching circuit, a model of each component needs to be individually created in such a manner that the model includes a parasitic capacitance component or parasitic inductance component and a loss resistance component in addition to a capacitance component or inductance component, and the value of each created component needs to be individually input. As a result, a process for creating an antenna model provided with a matching circuit and further including a parasitic capacitance component, a parasitic inductance component, and a loss resistance component included in a matching element becomes complicated.
Moreover, in the conventional antenna designing process, a matching circuit to be added to an antenna needs to be configured by evaluating whether or not a simulation result conforms to a desired standard, and hence the matching circuit is unable to be determined efficiently.